Method for pre treatment verification in radiation therapy

ABSTRACT

The present invention relates to a method to measure dose distribution in a patient-shaped phantom with high accuracy. The invention consists of a method of measuring dose distribution in a phantom for radiation therapy treatment verification, a detector configuration in such a phantom, detector improvement and measurement methodology to enable application of correction factors in an accurate way.

TECHNICAL FIELD

The present invention relates generally to radiation therapy treatmentverification. In particular the invention pertains to methods, adetector configurations, a detector and a computer readable medium forverifying that a patient specific cancer treatment using radiationtherapy, and in particular intensity modulated radiation therapy, isdelivered as planned.

BACKGROUND OF THE INVENTION

Radiotherapy has been used to treat cancer in the human body since early1900. Even though radiation of cancer tumours is known to be efficient,mortality rate for many cancers remained virtually unchanged for a longtime. The major reasons for this have been the inability to control theprimary tumour or the occurrence of metastases. Only by improving thelocal control may the treatment be more effective. In the last yearsTreatment Planning Systems, TPS, in Radiation Therapy have developedextensively and is now able to take into account the anatomy of thespecific patient and in a time efficient way plan a more optimisedtreatment for each individual patient, homogenous dose to the target andminimum dose to risk-organs.

The treatment technique to deliver this optimised treatment is morecomplicated than conventional treatments because each field must bemodulated laterally in intensity and thereby compensate for the patientscontour and anatomic heterogeneity, the technique is calledIMRT—Intensity Modulated Radiation Therapy. The delivery can be doneusing compensators, filters individually made for each projection, thatreduce the intensity to a predefined level in each part of the field dueto attenuation of the primary photon beam. However when using severalprojections (4-8), each projection requiring individual compensators,this technique is time-consuming and requires a lot of effort.Additionally the attenuation of the beam in the filter causes unwantedchange of the beams spectral distribution, thereby complicating thewhole process. The most common way to deliver IMRT is therefore usingthe MLC (Multi Leaf Collimator) a device that consists of thin blocks(collimator-leafs) that can be individually positioned to block a smallpart of the field and thereby shape the beam in the lateral direction tovarious irregular shapes. In each projection the collimator-leafs aremoved during the treatment and thereby various part of the cross-sectionof the beam is irradiated during various times—the dose distribution ismodulated. The conformity of the dose distribution to the tumour can befurther improved using even more sophisticated techniques also changingthe projection while the beam is on e.g. ARC-therapy.

In conventional therapy it is sufficient to make periodic verificationon the level of the dose distribution on the central axis and in a fewpoints off-axis to verify the beam-symmetry and beam-flatness. The newtreatment technique is complicated and involves the transfer ofinformation between several systems and therapy system sub-modules, andthe cross section of the beam is individual for each projection on eachpatient, thereby extended quality assurance is required.

The fundamental of IMRT, building-up the dose in the field by blockingsome parts of it longer than other often increases the beam-on time, andthereby the dose to the area outside the field increases. In IMRTaccurate measurement of the dose to the areas outside the field isthereby more important than in conventional treatment, a requirementthat further increases the demands in the measurement process.

Good quality control procedures in a radiation therapy clinic treatingwith IMRT technique includes:

-   -   Machine specific quality assurance e.g. stability check of the        dose rate, time for the treatment system to stabilise,        mechanical QA of the MLC etc. before the treatment machine is        accepted to be used for treatments.    -   Pre treatment verification—Measurement performed on each        individual treatment plan, before the patient is given the first        treatment fraction, to verify the ability to deliver the        treatment accurately.    -   Patient dosimetry or in vivo dosimetry—Verification of the        delivered dose to the patient during the actual treatment, see        the Swedish patent application 0201371-2.

Pre treatment verification can be done for each individual projectionusing a 2D detector in a flat phantom positioned perpendicular to thebeam or it can be done for one treatment occasion including allprojections using a body phantom with detectors. Both methods haveimplementations using traditional measurement techniques and both ofthem have important limitations both in methodology and in measurementaccuracy.

The shortcoming of the first method is:

-   -   Complicated and time-consuming to verify each projection        individually rather than the total contribution from all        projections in one comparison.    -   Unnecessary efforts invested in correcting minor errors per        projection that would have shown to be neglectable if all        projections could be totalised.    -   The verification excludes errors in gantry angle and collimator        rotation since the device is either attached to the gantry or        the gantry rotation is not used during verification.    -   It is not useful in ARC-therapy (described above).    -   Lack of time resolution in the measurements disables the        possibility to analyse the course of a measured deviation, e.g.        in sub fields or segments of a field without updated        measurement. Additionally there is no possibility to distinguish        whether a dose is delivered when expected during the respiratory        cycle.

The first method has been implemented in a product, MapCheck availablefrom Sun Nuclear INC., consisting of a matrix of diodes where eachdetector integrates the dose during the delivery of one projection.Measuring at one depth in the same beam direction simplifies most of therequirements on the detector to similar as in traditional measurements.However the requirement to measure with high accuracy outside theprimary field, described above, raises several demands on the detectorsand one of the hardest to fulfil for semiconductors is energyindependency.

The second method simulates the patient on the couch using a body shapedor head-and-neck shaped plastic-phantom, see U.S. Pat. No. 6,364,529[MED TEC IOWA INC (US)), with some kind of detectors inserted into it.The phantom that is placed on the couch, without connection to thegantry rotation, can be irradiated similar to a patient in any relevantprojection. Thereby the delivered dose from all projections can bemeasurement in any point inside the phantom. Error in delivery e.g. inMLC position, gantry angle, collimator rotation etc. will cause similardose discrepancy in the phantom as in the patient.

Until now this method has been used with radiological films placedinside the phantom in the direction of the beam and a fewpoint-detectors. The film measures thereby in 2-dimension (2D) along thebeam with high spatial resolution. However, across the field, where thebeam is modulated, the method is limited to measure along the film (1D).The main reason for the orientation of the film is the shortcoming offilm as a detector. The response of radiological films depends onseveral parameters e.g. direction of radiation, energy, pressure (thepressure on the film at exposure), development process, fading,linearity etc. Additionally, film is an integrating detector and therebythe film-data has no time resolution and thereby analyses of the causeof a deviation between measurement and the treatment plan often becomemore or less impossible. Ideal point detectors would measure thepoint-dose accurately, however a few point detectors will not enableverification of the intensity modulated beam in the various projections.Ideal detectors do not exist and currently used measuring methods thathave no time resolution and/or synchronisation or documentation to thetreatment phase makes it impossible to apply relevant corrections to themeasurement and thereby improve the result.

Direct measuring detectors currently used on the radiotherapy market areionisation chambers and semiconductors. The ionisation chambers has ingeneral a better long term stability than semiconductors. However, thespatial resolution of the ionisation chamber is rather limited, normallyabout 3-4 mm, which is a major limitation in the applications discussedherein.

Nearly 10 years ago scintillation detectors was proposed for radiationtherapy, but is has however not been possible to make this technologywork in practice. One of the main reasons is that the PM tube orphotodiode that is used to convert the light to an electric signal mustbe kept out of the primary beam and the fibre optics used to connect thescintillation detectors to the PM tube or the photodiode createsscintillations as well. Proposals using dummy fibre optics has beenpresented, but the underlying technical problems has not been possibleto solve.

Semiconductors are mainly diodes or MOSFET detectors. Both these typesare based on silicon and thereby they have the same or similar energydependency and both have a high specific efficiency measuring radiation,which is an important parameter when measuring small doses. In“Investigation of the use of MOSFET for clinical IMRT dosimetricverification” Chuang, F. Cynthia, et. al., Med. Phys. 29(6), June 2002,a MOSFET detector system for IMRT verification is disclosed. This systemprovides for an easy calibration and an instantaneous read-out of testresults but shows reproducibility, linearity, energy and angularresponses similar to that of conventional dosimeters. The major drawbackwith the disclosed MOSFET detector system is however the limitedlifespan of the detectors, which is mainly caused by radiation damage.Normally, the tolerance against radiation damage is approximately 200 Gyfor a MOSFET detector. Moreover, the absorbed dose in a MOSFET can beread directly or after use, but not in real time applications.

Diodes are very reliable detectors with a high tolerance againstradiation damage that exceeds 200000 Gy, i.e. approximately 1000 timeshigher in comparison with a MOSFET. Both MOSFET detectors and ionisationchambers require a bias, which complicates a system with an extensivenumber of detectors. Diodes are generally very reliable detectors andare used in many applications, e.g. integrating measurements as in vivodosimetry and output factor measurements in small fields respectivelyreal-time measurement, e.g. relative measurement in water phantoms. Themain limitation is the energy dependence and the long term stabilityeven though the latter has been improved during the recent years.

BRIEF DESCRIPTION OF THE INVENTION

Thus, an object of the present invention is to provide an efficientpre-treatment measurement method that sufficiently and accuratelyverifies the dose distribution from a complete treatment fraction (allbeam projections) to be delivered to a patient.

It is a further object of the present invention is to provide tools tofind the causes of deviations compared to a treatment plan.

These and other objects are achieved according to the present inventionby providing methods, a computer readable medium, and a detectorconfiguration having the features defined in the independent claims.Preferable embodiments of the invention are characterised by thedependent claims.

According to a first aspect of the present invention, there is provideda method of measuring dose distribution in a phantom for radiationtherapy treatment verification, wherein at least two detector planes arearranged in said phantom in a non-parallel manner, each plane beingprovided with a plurality of diode detectors, wherein said phantom isirradiated using a patient specific treatment. The method comprises thesteps of obtaining information regarding the dose distribution insidesaid phantom by performing measurements using said detectors, dividingthe measurements in time-intervals; and using said information in thetreatment verification.

According to a second aspect of the present invention, there is provideda detector configuration in a phantom suitable for radiation therapy,comprising at least two detector planes provided with a plurality ofdiode detectors for measuring irradiation in said phantom, saidirradiation being delivered using a patient specific treatment. Theplanes are arranged in a non-parallel manner, wherein said detectors hasa thickness in a range less than the range of the electrons of themaximum energy in the range where the dependency is significant.

According to a further aspect of the invention, there is provided acomputer readable medium comprising instructions for bringing a computerto perform the method according to the first aspect of the invention.

Thus, the invention is based on the idea of a configuration of diodedetectors in two or more non-parallel planes in a phantom e.g.body-phantom (without connection to the gantry rotation, i.e. therotation of the device applying the irradation), wherein the measurementis divided in time-intervals. The special configuration of the detectorsmakes it possible to verify the intensity modulation across the beam inany beam projection and at the same time totalise the dose from allprojections in the fixed measurement-points in the phantom. In addition,the overall measurement accuracy is significantly improved by dividingthe measurement in time-intervals. Obviously, this is clear advantagesof the present invention compared to existing solutions. For example, aplacement of the detectors in a 3D matrix would require an extensivelyincreased number of detectors, which, in turn, would entail very highcosts and which also would require significant processing times in orderto process the information or data obtained from the detectors duringthe measurements. Furthermore, the division into time-interval enablesuse of individual correction factors for each time interval. Inaddition, this facilitates an evaluation of discrepancies in dynamicfields and/or ARC-therapy and to reduce the directional and/or energydependency of the detectors.

Preferably, the information obtained at the measurements of the dosedistribution in the phantom is used at IMRT treatment verification.

The length of a time interval depends on the IMRT technique used as wellas the size and change of correction factors. The case when a detectorgoes from being inside the field to be outside is most important and ittakes approximately 100 msek. Typical values for the time intervals arethereby in the range from 10 usek (one pulse, gating) to 100 msek. Thus,the time intervals are, i.a. defined from the required overall accuracyin dose determination. The dose contribution in each time interval canbe totalised for the whole treatment as a first step to verify thecomplete treatment delivery, discrepancies can then be further analysedby comparisons at each field (projection) and sub fields. By totalisingthe dose from the intervals in various ways, a complete fraction, perfield (projection), per sub field etc. the analyse can get deeper whenrequired still using the same measurement data and thereby no updatedmeasurement must be done which will save time and also make it possibleto find intermittent errors.

According to an embodiment of the present invention, the dosemeasurements are synchronized with the delivered accelerator pulses.These are stored with the information on the current parameters of thetreatment unit, e.g. projection. Measurements in short time-intervals(<100 msec) require a high detection efficiency per unit volumeespecially when the demands simultaneously is high on the spatialresolution, typically 1 mm. Using a silicon diode with the requiredsize, the signal level generated outside the primary field is in therange of pA. Such small currents are difficult to handle with highaccuracy and the electronic noise becomes significant. The therapysystem delivers the dose in pulses at a frequency between 50 and 1000Hz, each pulse having a length of less than 10 μsec and thereby theradiation is distributed during less than 10% of the time. By measuringonly during the pulses, i.e. synchronizing the dose measurements withthe delivered pulses, the noise can be reduced to a minimum and the dosecan be measured at the required accuracy.

According to another embodiment of the present invention, themeasurements are synchronized with the respiratory cycle of the patientfor which the patent specific treatment is intended. External signalsobtained by means of, for example, an X-ray unit, which indicates thephase of the respiratory cycle, e.g. used to turn the irradiation beamon and off, is stored with each measurement. Thereby, the dose deliveredin the various phases of the respiratory cycle can be determined. Themeasured dose per pulse can be grouped in various time-intervals due tothe requirements at the specific set-up. Furthermore, the IMRT treatmenttechnique increases the requirement on reproducible patient positioning.The synchronisation with the respiratory cycle, so called respiratorygating, decreases the irradiation of healthy tissue. That is, themargins that are added in order to ensure that the tumour without doubtis within the applied field can be reduced by using respiratory gatingand thereby the proportion of irradiated healthy tissue can bedecreased.

In addition, synchronizing measurements with delivered acceleratorpulses can be utilized in combination with synchronization between themeasurements and the respiratory pulses to verify that the dose is givenat the correct phase of the respiratory cycle of the patient.

According to embodiments of the present invention, correction factorsare calculated according toCorr_(n,f,seg-n,f,p,t(i),t(i+1)) =C _(dir) *C _(depth) *C _(pos)  (1)orCorr_(n,f,seg-n,f,p,t(i),t(i+1)) =C _(dir) +C _(depth) +C _(pos)  (2)

-   Corr_(n, f, seg-n, f, p, t(i), t(i+1)) The correction factor to be    used with detector-element n, in the sub-field, f in the phantom,    correcting the measured dose integrated from time t(i) until t(i+1)    to achieve the dose in the point of the detector n location.-   C_(dir) Factor correcting for any directional dependency in the    detector-   C_(depth) Factor correcting for any depth (energy and or dose rate)    dependency in the detector-   C_(pos) Factor correcting for any position (in primary beam, outside    primary beam, edge of primary beam etc.) dependency in the detector

According to embodiments of the present invention, each detector planemay be provided with detectors having a thickness in a range less thanthe range of the electrons of the maximum energy in the range where thedependency is significant. (<200 keV corresponding to approximately 200μm in silicon). Thereby, the energy and/or directional dependency of thedetectors are reduced significantly, which mainly is caused bydifferences in the photons mass-attenuation in the detectors comparedwith the media they are arranged in.

As realized by the person skilled in the art, the method of the presentinvention, as well as preferred embodiments thereof, are suitable torealize or implement as a computer program or a computer readablemedium, preferably within the contents of the control and measurementsystem of the radiotherapy device, and thereby using the processor andstorage means available there. Alternatively it may be implemented in astand-alone unit comprising the necessary equipment such as a centralprocessing unit CPU performing the steps of the method according to theinvention. This is performed with the aid of a dedicated computerprogram, which is stored in the program memory. It is to be understoodthat the computer program may also be run on a general purposeindustrial computer instead of a specially adapted computer.

Further objects and advantages of the invention will be discussed belowby means of exemplifying embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed description, reference will be made to theaccompanying drawings, of which

FIG. 1 a schematically shows a treatment machine to which a phantom isarranged, which in turn is provided with detectors,

FIG. 1 b schematically shows the arrangement of FIG. 1 a but with ahuman body instead of the phantom,

FIG. 2. schematically shows a typical example on a body shaped phantomwith two crossing planes with detectors in a special arrangement tooptimise the number of detectors.

FIG. 3. schematically shows various beam directions (projections)towards the body shaped phantom.

FIGS. 4 a-4 c. schematically shows different examples of patterns forarranging the detectors on the detector planes.

FIG. 5 schematically shows an embodiment of the method of measuring dosedistribution in a phantom for radiation therapy treatment verificationaccording to the present invention.

FIG. 6 schematically shows an embodiment of a system in which the methodof measuring dose distribution in a phantom for radiation therapytreatment verification according to the present invention can beemployed.

DETAILED DESCRIPTION OF THE INVENTION

A radiotherapy device (gantry) utilised for treating tumours withradiation are shown schematically in FIGS. 1 a and 1 b and is generallydenoted with reference numeral 10. The device comprises a radiotherapysystem capable of emitting a beam 12 of electrons or photons from atreatment head. The radiotherapy system is provided with conventionalfield-shaping device (not shown), for example an MLC, for allowing thelateral shape of the beam to be altered so as to shield off non-affectedareas of the body, intensity modulate the beam and concentrate the beamto the tumour. The radiotherapy system comprises control and measurementmeans (not shown) including processor and storage means, for example, acentral processing unit CPU for performing the steps of the methodaccording to the invention.

A treatment couch 16 is arranged for a patient 14 to lie on, see FIG. 1b. The couch can rotate around a vertical axis, and move horizontally,vertically and longitudinally in order to place the area to be treatedof the patient in the area of the beam.

Further, the method according to the invention utilises detectors placedinside a phantom e.g. body-phantom in a way that reduces the requirednumber of detectors to a minimum and still enables verification of theintensity modulated beams in all projections and simultaneously measurestotal dose distribution from all beam-projections in fixed points in thetreated volume.

Preferably, the detectors are arranged in two or more non-parallelplanes arranged in such a way that the crossing point of the planes islocated in the vicinity of the rotation point of the treatment machine,preferably within 5 cm from the rotation point, and that either of thedetector-planes covers the whole cross section of the beam in anyprojection FIGS. 2 and 3. FIG. 2 shows detector-planes 20 and 21 placedinside the phantom 22 with detectors configured in lines 23 and in anarea 24 and FIG. 3 is an illustration of typical beam directions 30irradiating the phantom from different projections.

FIGS. 4 a-4 c schematically shows different examples of detectorpatterns on detector planes 40, 40′ and 40″, respectively. FIG. 4 ashows a detector plane 40 provided with a number of detectors lines 41,each line 41 represents an one dimensional (1D) array 42 of detectors 44or a zigzag pattern 43 of detectors 44, FIG. 4 b shows a detector plane40′ provided with detectors 44 arranged according to a two dimensionalarray, i.e. a matrix of detectors 44, and FIG. 4 c shows a detectorplane 40″ provided with detectors 44 arranged according to combinationof the configurations shown in FIGS. 4 a and 4 b.

The method according to the present invention is aimed to utilise theabove-mentioned equipment in order to enable measurement andverification of dose delivery in radiotherapy treatment, in particularprior to applying the treatment on the patient (pre treatmentverification). The measured dose distribution is aimed to be comparedwith the dose distribution from the planned treatment of the specificpatient after recalculating it to a similar phantom as the measurementphantom.

A typical sequence from diagnostics to IMRT treatment are describedbelow:

-   -   An individual treatment plan for the patient is made using a        Treatment Planning System (TPS). The anatomy of the patient has        first been defined using diagnostic equipment e.g. CT,        Computerised Tomography and the radiation characteristics of the        treatment device is defined generally by measurements both        imported in the TPS. The target-volume and risk-organs are        defined and then the optimum plan for the treatment is made        where criteria as maximum dose to the risk-organs and the        minimum dose to the target etc. is used. The outcome of the plan        is information that will be used by the treatment machine to set        projections, beam modality, field shapes and movement of the        MLC-leaves etc.    -   The patient specific treatment plan, in the TPS, is applied on a        phantom, suitable for dose measurements, and the dose        distribution inside the phantom, using the patient specific        treatment, is calculated.    -   Prior to treatment, a physical phantom, identical to the one        simulated in the calculation, is irradiated using the patient        specific treatment. The dose distribution inside the phantom is        measured and integrated per projection and for all projections,        complete fraction.    -   The measured and the calculated dose distribution are compared        to verify the delivery of the patient specific treatment.

Accordingly, information regarding the dose distribution inside saidphantom is obtained by performing measurements using the detectors,which information is used in the treatment verification and/or stored.

Turning now to FIG. 5, the general principles of the method of measuringdose distribution in a phantom for radiation therapy treatmentverification according to the present invention will be described.

Detector planes comprising a number of diode detectors arrangedaccording to a detector pattern, for example, one of the patterns shownin one of FIGS. 4 a-4 c, are placed in the phantom. The phantomincluding the detectors is placed in the isocenter (rotation centre) ofthe treatment machine and is aligned using the positioning lasers in thetreatment room. The measurements electronics is connected to a PClocated in the control room, see FIG. 6. A connection between thetreatment machine and the measurements electronics and/or via, forexample, a LAN to the controlling PC might be established to synchronizethe measurement and the delivery of the treatment. At step 52, thepre-treatment verification is initiated, i.e. the phantom is irradiatedaccording to the patient specific treatment. Then, at step 53, themeasurement data from each detector is collected for each time-interval.By using individual correction factors for each time-interval, asignificant enhancement of the measurement accuracy is accomplished, seebelow. At step 54, after completed irradiation, or simultaneously, thedata is processed and corrected using equations (1) or (2). Thereafter,at step 55, the total dose delivered to each detector is calculated.Further, the planed treatment is imported to the PC-SW. Then, at step56, the measured dose is compared with the calculated dose. If thedeviation exceeds a certain action-level, the calculated dosedistribution in the treatment plan in each projection might be importedand the dose is recalculated for each projection for comparison. If itis required, the comparison can be performed on sub-fields (i.e. a partof a projection).

With respect now to FIG. 6, an embodiment of a system in which theinvention can be implemented is shown. A reading unit 60 comprising amicroprocessor 62 and storage means 63 is connected to detector planes61, see FIGS. 2, and 4 a-4 c. The reading unit 60 is arranged to recordor measure the absorbed dose of each detector. The storage means 63 mayinclude a random access memory (RAM) and/or a non-volatile memory suchas read-only memory (ROM). As will be appreciated by one of ordinaryskill in the art, storage means may include various types of physicaldevices for temporary and/or persistent storage of data which includessolid state, magnetic, optical and combination devices. For example, thestorage means may be implemented using one or more physical devices suchas DRAM, PROMS, EPROMS, EEPROMS, flash memory, and the like. Inaddition, the reading unit 60 is provided with an input for receivingsignals from the therapy system 65 and an input for receiving signalsfrom external devices 66, such as an X-ray unit. By using the signalsfrom the therapy system the measurements can be synchronized with thedelivered dose pulses. Moreover, by connecting an external device, suchas an X-ray unit or a laser sensor, monitoring the respiratory cycle,the measurements can be synchronized with the respiratory cycle.Thereby, respiratory gating can be performed in order to decrease theirradiation of healthy tissue. That is, the margins that are added inorder to ensure that the tumour without doubt is within the appliedfield can be reduced by using respiratory gating and thereby theproportion of irradiated healthy tissue can be decreased. Furthermore,the reading unit 60 is connected to communication means 64 for wirelesscommunication of, for example, measurement data to an externalprocessing unit 67, for example, an PC. In this embodiment, thecommunication means 64 is a LAN connection. The method according to theinvention may be implemented in the control and measurement system ofthe radiotherapy device. Alternatively it may be implemented in astand-alone unit comprising the necessary equipment such as a centralprocessing unit CPU performing the steps of the method according to theinvention, for example, the personal computer 67. This is performed withthe aid of a dedicated computer program, which is stored in the programmemory. It is to be understood that the computer program may also be runon a general purpose industrial computer instead of a specially adaptedcomputer.

The software includes computer program code elements or software codeportions that make the computer perform the method using equations,algorithms, data and calculations described herein. A part of theprogram may be stored in a processor as above, but also in a ROM, RAM,PROM or EPROM chip or similar. The program in part or in whole may alsobe stored on, or in, other suitable computer readable medium such as amagnetic disk, CD-ROM or DVD disk, hard disk, magneto-optical memorystorage means, in volatile memory, in flash memory, as firmware, orstored on a data server.

According to an embodiment of the present invention, the dosemeasurements are synchronized with the delivered accelerator pulses.These measurements are stored with the information on the currentparameters of the treatment unit, e.g. projection. Measurements in shorttime-intervals (<100 msec) require a high detection efficiency per unitvolume especially when the demands simultaneously is high on the spatialresolution, typically 1 mm. Using a silicon diode with the required sizethe signal level generated outside the primary field is in the range ofpA. Such small currents are difficult to handle with high accuracy andthe electronic noise becomes significant. The therapy system deliversthe dose in pulses at a frequency between 50 and 1000 Hz, each pulsehaving a length of less than 10 μsec and thereby the radiation isdistributed during less than 10% of the time. By measuring only duringthe pulses, i.e. synchronizing the dose measurements with the deliveredpulses, the noise can be reduced to a minimum and the dose can bemeasured at the required accuracy.

According to another embodiment of the present invention, themeasurements are synchronized with the respiratory cycle of the patientfor which the patent specific treatment is intended. External signalsobtained by means of, for example, an X-ray unit, which indicates thephase of the respiratory cycle, e.g. used to turn the irradiation beamon and off, is stored with each measurement. Thereby, the dose deliveredin the various phases of the respiratory cycle can be determined. Themeasured dose per pulse can be grouped in various time-interval due tothe requirements at the specific set-up. Furthermore, the IMRT treatmenttechnique increases the requirement on reproducible patient positioning.The synchronisation with the respiratory cycle, so called respiratorygating, decreases the irradiation of healthy tissue. That is, themargins that are added in order to ensure that the tumour without doubtis within the applied field can be reduced by using respiratory gatingand thereby the proportion of irradiated healthy tissue can bedecreased.

In addition, synchronizing measurements with delivered acceleratorpulses can be utilized in combination with synchronization between themeasurements and the respiratory pulses to verify that the dose is givenat the correct phase of the respiratory cycle of the patient.

As indicated above, a further enhancement of the measurement accuracycan be accomplished by dividing the measurements in short time-intervalsand using individual correction factors for each time-interval. Thelength of a time interval depends on the IMRT technique used as well asthe size and change of the correction factors. Thus, the time intervalsare i.a. defined from the required overall accuracy in the dosedetermination. The dose contribution in each time interval can betotalised for the whole treatment as a first step to verify the completetreatment delivery, discrepancies can then be further analysed bycomparisons at each field (projection) and sub fields. According topreferred embodiments of the present invention, the correction factorsare calculated according toCorr_(n,f,seg-n,f,p,t(i),t(i+1)) =C _(dir) *C _(depth) *C _(pos)  (1)orCorr_(n,f,seg-n,f,p,t(i),t(i+1)) =C _(dir) +C _(depth) +C _(pos)  (2)

-   Corr_(n, f, seg-n, f, p, t(i), t(i+1)) The correction factor to be    used with detector-element n, in the sub-field, f in the phantom,    correcting the measured dose integrated from time t(i) until t(i+1)    to achieve the dose in the point of the detector n location.-   C_(dir) Factor correcting for any directional dependency in the    detector-   C_(depth) Factor correcting for any depth (energy and or dose rate)    dependency in the detector-   C_(pos) Factor correcting for any position (in primary beam, outside    primary beam, edge of primary beam etc.) dependency in the detector

Which one of (1) or (2) that is selected depends on how C_(dir),C_(depth) and C_(pos) were obtained. Preferably, equation (1) is usedwhen the correction factors are accepted to be independent of each otherand, accordingly, can be obtained individually. Obtaining the correctionfactors using this equation is time efficient. Preferably, equation (2)is used if each combination of factors are to be measured. This methodprovides very accurate results.

If the diode-material differs in mass-density or electron-density fromthe phantom it might be selected thin at least in one dimension toreduce energy and directional dependency. Preferably, the detector ismade thinner than the range of the electrons of the maximum energy inthe range where the dependency is significant, e.g. for Si-detector inwater the energy dependency is documented for photons with energy lessthan 200 keV where the electron range in Si is 200 um. The directionaldependency is improved when Silicon is thinner than 500 um.

-   -   For a detector where all material except the sensitive part, is        similar in mass-attenuation as the media it will measure in,        only the sensitive part have to be thinner than the range of the        electrons, for the maximum energy where the dependency is        significant, in order to reduce the energy dependency.    -   For a detector where both the sensitive part and the surrounding        material differ in mass-attenuation compared to the media it is        arranged in, the sensitive part and the material that differs        must be thin enough to reduce the energy dependency.

In addition, the “thin detector”, i.e. a detector having a thicknessmade thinner than the range of the electrons of the maximum energy inthe range where the dependency is significant, can preferably be used inseveral other applications such as: Water phantom dosimetry and in vivodosimetry during Brachy therapy in Radio therapy. Water phantomdosimetry is performed using fixed detectors or detectors placed on aservo mechanism in a phantom filled with water. The system has severalapplications: acceptance tests of a treatment machine—generalmeasurement of the dose distribution from the treatment machine; andmeasurement of the dose distribution in 3D. In vivo dosimetry duringBrachy therapy (radioactive sources inserted into the human body)incorporates measurements inside the human body, interstitial or intracavity e.g. thrachea, uterus, rectum, and bladder

It is to be understood that the above description of the invention andthe accompanying drawings is to be regarded as a non-limiting examplethereof and that the scope of protection is defined by the appendedpatent claims.

1. Method of measuring dose distribution in a phantom for radiationtherapy treatment verification, wherein at least two detector planes arearranged in said phantom in a non-parallel manner, each plane beingprovided with a plurality of diode detectors, wherein said phantom isirradiated using a patient specific treatment, comprising the steps ofobtaining information regarding the dose distribution inside saidphantom by performing measurements using said detectors; dividing themeasurements in time-intervals, each time-interval having maximum lengthof approximately 100 msec; and using said information in the treatmentverification.
 2. Method according to claim 1, wherein the informationobtained by means of said measurements is used for IMRT verification. 3.Method according to claim 1, wherein said irradiation of the phantomcomprises delivering dose pulses, further comprising the step ofsynchronizing the measurements with said delivered doses.
 4. Methodaccording to claim 1, further comprising the steps of: synchronizing themeasurements with a respiratory cycle of the patient for which thepatent specific treatment is intended; and determining the dosedelivered in the various phases of the respiratory cycle.
 5. Methodaccording to claim 1, further comprising the step of storing the datafor each specific time-interval for measurements in said phantom. 6.Method according to claim 1, further comprising the step of calculatingcorrection factors for each time-interval using said obtainedinformation regarding the dose distribution inside said phantom. 7.Method according to claim 6, wherein the correction factors arecalculated according toCorn,f,seg-n,p,t(i),t(i+1)=Cdir*Cdepth*Cpos where Corrn, f, seg-n, p,t(i), t(i+1) The correction factor to be used with detector element n,in the sub field f in the phantom, correcting the measured doseintegrated from time t(i) until t(i+1) to achieve the dose in the pointof location of detector n Cdir Factor correcting for any directionaldependence in detector n Cdepth Factor correcting for any depth (energyand/or dose rate) in detector n Cpos Factor correcting for any position(in primary beam, outside primary beam, edge of primary beam, etc.)dependency in detector n.
 8. Method according to claim 6, wherein thecorrection factors are calculated according toCorrn,f,seg-n,p,t(i),t(i+1)=Cdir+Cdepth+Cpos where Corn, f, seg-n, p,t(i), t(i+1) The correction factor to be used with detector element n,in the sub field f in the phantom, correcting the measured doseintegrated from time t(i) until t(i+1) to achieve the dose in the pointof location of detector n Cdir Factor correcting for any directionaldependence in detector n Cdepth Factor correcting for any depth (energyand/or dose rate) in detector n Cpos Factor correcting for any position(in primary beam, outside primary beam, edge of primary beam, etc.)dependency in detector n.
 9. Method according to claim 1, wherein thedetector planes are arranged such that for each gantry angle projection,either of said non-parallel planes intersects with all parts of theradiation beam or sub-beams.
 10. Method according to claim 1, whereineach detector plane is provided with detectors having a thickness in arange less than the range of the electrons of the maximum energy in therange where the dependency is significant.
 11. Method of measuring dosedistribution in a phantom for radiation therapy treatment verification,wherein detector planes are arranged in said phantom, each plane beingprovided with a plurality of diode detectors, wherein said phantom isirradiated using a patient specific treatment, comprising the steps ofobtaining information regarding the dose distribution inside saidphantom by performing measurements using said detectors; dividing themeasurements in time-intervals, each time-interval having maximum lengthof approximately 100 msec; synchronizing the measurements with arespiratory cycle of the patient for which the patent specific treatmentis intended; determining the dose delivered in the various phases of therespiratory cycle; and using said information in the treatmentverification.
 12. Method according to claim 11, wherein at least twodetector planes are arranged in said phantom in a non-parallel manner.13. (canceled)
 14. Use of a detector configuration arranged in a phantomsuitable for radiation therapy in a method according to claim 1, saidconfiguration comprising at least two detector planes provided with aplurality of diode detectors for measuring irradiation in said phantom,said irradiation being delivered using a patient specific treatment,wherein said planes being arranged in a non-parallel manner, whereinsaid detectors has a thickness in a range less than the range of theelectrons of the maximum energy in the range where the dependency issignificant.
 15. Detector configuration according to claim 14, whereinsaid non-parallel planes are arranged such that, for each gantry angelprojection, either of said planes intersects with all parts of theradiation beam or sub-beams.
 16. Use of a diode detector arranged with athickness in a range less than the range of the electrons of the maximumenergy in the range where the dependency is significant in a methodaccording to claim
 1. 17. Diode detector according to claim 16, whereinsaid detector is used in water phantom dosimetry or in vivo dosimetryduring Brachy therapy in Radio therapy.
 18. Computer readable mediumcomprising instructions for bringing a computer to perform the steps ofthe method according to claim 1.